i) Field of the Invention
This invention relates to the production of paper and paper board and to a polymer additive for use in such production.
The invention also relates to processes for producing the polymer additive; a papermaking stock containing the polymer additive; a paper sheet derived from the papermaking stock containing the polymer additive; and a method of enhancing retention of components of a papermaking stock in a sheet formed from the stock.
ii) Description of the Prior Art
1) Sheet Formation and the Role of Retention Aids
During the fabrication of paper, a flat jet of a dilute fibrous suspension is injected onto the surface of a specially designed textile called a forming fabric, or into a converging gap formed by two forming fabrics. The bulk of the water is rapidly drained through one or two fabrics, while a large portion of the suspended solids, such as fibres, fines and filler, is retained by the fabric to form a sheet. Ideally, all the solid material dispersed between the fabrics should be retained in the sheet; however, a portion of the solids, especially very small particles and colloidal material escapes through the interstices of the forming fabrics. Retention is thus always less than perfect and for light grades of paper and tissue, it usually varies between about 20% and 80%. When the retention is low, a large amount of dispersed material is repeatedly circulated with whitewater before it is finally retained in the sheet. Some of the dispersed solids are not retained and, with the excessive whitewater proceed to the effluent treatment. Only a fraction of a second is usually available for water drainage on rapidly-operating, modern paper machines. Therefore, for good machine operation it is critical that the drainage occurs rapidly. However, during rapid drainage a high shear stress occurs in the forming zone, which tends to separate the particles of fillers and fines from the fibres, and thus impairing their retention.
The primary component of mechanical pulps is cellulosic fibres, but the pulps also contain about 30% of small wood debris usually referred to as fines. Furthermore, mineral pigments of small particle size are often used as fillers, in amounts ranging from a few percent to over 40% of sheet mass. These fillers are added to improve the printing quality of the paper, and to reduce its cost. The fines and fillers are too small to be retained on the forming fabrics by filtration. In the absence of chemical additives, a large proportion of these materials passes through the forming fabric and recirculates in the white water loop. Poor retention thus causes the loss of valuable papermaking material, impairs product quality and increases the cost of both production and waste water treatment.
A common papermaking practice includes the addition of polymeric materials (sometimes called retention aids) to the fibrous suspension in order to improve the retention of fines and fillers. The retention aids are capable of being adsorbed on to the surface of fines and fillers, causing the coagulation of fine particles into larger conglomerates and eventually, their adsorption onto the surface of pulp fibres. Polymeric additives described in the literature and available from various suppliers are used alone or with small molecular weight co-factors, sometimes also with one or two additional polymeric components, or with a mineral powder.
As with other natural fibres, pulp fibres are negatively charged. Many retention aids are positively charged polymers which are adsorbed onto the negatively charged fibres via electrostatic interactions. These polymers serve to improve the retention of fines and fillers during forming, and facilitate water drainage and some polymers are also used to improve product properties such as the strength of dry or rewetted product, or the rate of water or oil absorption.
Some water soluble polymers have a high density of cationic charge and are designed to reduce the negative charge of pulp components. One example of such polymers is poly(diallyl-dimethylammonium chloride), known as polydadmac [D. Horn and F. Linhart, in Paper Chemistry, Ch. 5 2nd Ed. by I. C. Roberts, Blackie Academic and Professional, London 1996.]. Other water-soluble polymers such as cationic starches [U.S. Pat. No. 2,768,162 (1956)] have only a low cationic charge, which improves their retention in the fibrous sheet, although the charge might not be sufficiently high to make these polymers act as good retention aids. Sizing agents, which are usually dispersed hydrophobic compounds designed to impede water absorption, are often applied as complexes or micelles with cationic materials, which cause their deposition on the negatively charge fibres [Can. Patent 1,247,810 (1989)]. The electrostatic absorption is also used for the retention of dyes and other papermaking additives.
2) Retention of Mechanical Pulps
Retention based on electrostatic interactions can be efficient for chemical pulps, which are composed of relatively pure cellulose, as most of the lignin and hemicellulose originally present in the wood is eliminated during pulping and bleaching and carry only a mild negative charge. Furthermore bleached chemical pulps are washed during their preparation and do not contain significant amount of dissolved and colloidal substances. By contrast, mechanical pulps contain almost all of the original wood mass, including almost all the hemicellulose and lignin and carry a much greater negative charge.
When wood is disintegrated by grinding or refining, as much as a few percent of it is dissolved and about 30% of the wood mass is converted to small fragments called “fines.” It is well known that the fines carry larger negative charge than fibres. A large portion of dissolved components is hemicellulose, consisting of oligomers, which contain negatively charged carboxylic groups. Even higher negative charge is present in peroxide-bleached mechanical pulps, in which the alkaline peroxide treatment generates additional carboxylic groups especially by hydrolysing the ester groups in hemicellulose. High anionic charge is also found in mechanical pulps produced with a sulfite or bisulfite pre-treatment of the wood chips, as the sulphonation leads to the formation of sulphonic acid groups. Strongly negatively charged dissolved or finely dispersed materials react with the added cationic polymer and deplete it before it would have the opportunity to be absorbed onto the fibre. The resulting electrostatically neutral complex of cationic polymer and anionic pulp components might remain dissolved or dispersed in water rather than absorbed onto the fibres. Thus the high negative charge of mechanical pulps overwhelms the positive charges found on common retention aids, and greatly diminishes their efficiency.
A very large amount of cationic polymer would be required to neutralize all, or at least a substantial proportion of anionic charge in mechanical pulps. This would make the cost of polymer too expensive for a practical application. Therefore, alternative strategies have been developed for the retention aid used with mechanical pulps. Some retention aids are neutral and therefore hardly affected by the negatively charged dissolved or colloidal components. An example of such a retention system is polyethylene oxide, commonly referred to as PEO, which is usually used in combination with an enhancer that improves its absorption onto pulp [U.S. Pat. No. 5,824,192 (1998)]. Polyethylene oxide acts as a retention agent only if it has extremely high molecular weight. Typically the basis weight of PEO used for retention would be 3 to 8 millions Dalton. The preparation of polymers of such a high molecular weight is difficult and therefore the high molecular weight PEO is expensive. Another strategy is to use a large amount of inexpensive, highly-cationic material called “coagulant” to neutralize some of the “anionic trash,” before adding more-expensive retention polymer having a high molecular weight and low cationic charge density, called “flocculent.” In yet another strategy cationic polymer is used to flocculate the negatively charged pulp and agitation is applied to break these flocks into microflocks. Negatively charged pigment, referred to as “micro particles” is then used to cross-link these microflocks and to improve the retention [U.S. Pat. No. 6,007,679 (1999)].
3) Papermaking Additives Containing Primary Amino Groups
The cationic charge of many papermaking polymers is due to the presence of quaternary amino groups, which remain cationic at all values of solution pH, or tertiary amino groups which are cationic only in acidic solution, where the tertiary amino groups are protonated. There are only few papermaking additives that contain a significant proportion of primary amino groups.
Starches substituted with primary amino groups have been prepared using complicated procedures, which, if applied on a commercial scale, would make the products too expensive and therefore unsuitable for application as papermaking additives. Examples of such synthetic routes are described by F. Pancirolli and A. A. Houghton [UK patent 493,513 (1938)]. An alternative route for the production of starch additives containing primary amino groups was published recently [M. Antal, et. al., U.S. Pat. No. 6,455,661, (2002)]. The primary objective of this patent was to either eliminate the use of additive by modifying the pulp or to convert to papermaking additives inexpensive natural products such as starch. In particular, the objective was to prepare a starch derivative that could be used as additive with mechanical pulps, which deactivate most common cationic starches. Even after the modification these natural products have a relatively low charge density, 10 to 100 times lower than compounds that are described in this document. Therefore the primary amino starches are expected to be only moderately effective retention aids. Indeed such starch could potentially be used as a flocculant, in combination with a highly-charged, low molecular weigh polymer similar to those disclosed herein. To our knowledge no starch with primary amino group is commercially used as a papermaking additive.
Polyethylenimine is one polymer, which contains about ¼ of its amino groups in their primary form. Chitosan, derived from sea crustaceans is a primary amino group-containing carbohydrate that was used commercially as a papermaking additive [M. Laleg and I. I. Pikulik, Nordic Pulp and Paper Res. J., 7(4):174 (1992)]. The highly cationic natural polymer, chitosan, is a good retention additive when used with mechanical pulps and, compared with common cationic additives, resists better the deactivation by anionic charge of the pulp. Chitosan also increases the strength of freshly-formed wet webs, and of dried and then rewetted paper. These observations were rationalized by the presence of primary amino groups, which are capable of forming imino bonds with the carbonyl groups present at both ends of a glucose polymer and in lignin. Although chitosan derived from the shells of sea crustaceans found some application in papermaking, the material is too expensive for a wide-spread application. Different from chitosan most synthetic polymers derive their cationic charge from the presence of tertiary or quaternary ammonium groups. Tertiary amines or quaternary ammonium compounds can not form imino bonds with carbonyl groups [P. V. Sollenerger and R. B. Martin, Ch. 7 in The Chemistry of the Amino Group, Ed. by S. Patai, John Wiley and Sons, London (1968)]. Hemicelluloses modified chemically to contain amino groups, including primary amino groups were described in German literature [M. Antal, et. al., Das Papier, 51(5):223 (1997)].
Dendrimers were recently identified as effective flocculants for papermaking. Dendrimers are synthesized by adding to the central core several substituents, which are then chemically modified. In the subsequent reaction another series of substituents can be added onto each substituent added in the first reaction. The addition of substituents and their chemical modification can be repeated several times, each reaction sequence adding a new layer of substituents and forming a dendrimer of a “new generation.” [G. R. Newkome, J. Polymer Sci., Part A, Polymer Chemistry, 31 (1993), p. 611-651.] While many dendrimers were reported in literature those tested as papermaking additives contained primary amino substituents [M. Polyerari, et. al., Tappi J. vol. 2, No. 9, September 2003, pp 3-8]. Two chemical reactions are required to enlarge a molecule of dendrimer by a new layer of substituents, which makes dendrimers expensive. Indeed those that were investigated for application as papermaking additive were formed by only one or two reaction sequences and therefore had low molecular weight [Polypropyleneimine, Brabander-van der Berg E. M. M. and Meijer E. W., Angew. Chem. Intl. Ed. Engl. 32-38]. These dendrimers contained some primary amino groups, but were found too expensive for an application in paper production. To our knowledge from polymers containing primary amino groups only chitosan has found some rare and sporadic application in the paper industry, as the cost of known synthetic polymers having primary amino is too high for papermaking applications.